Documents ER-A-0544962 and WO 03/044499 disclose an infrared imager associated with optical measurement and reference filters that are placed successively on the imager's optical axis and which have bandwidths containing an absorption line of a desired gas (for measurement filters) or which are complementary to this absorption line (for reference filters). The bottom of the observed area is used as an infrared source and the presence of a desired gas is demonstrated by applying differential processing to infrared images taken through the filters, whereby the processing allows for the concentration of the detected gas to be calculated.
Conveniently, a set of measurement and reference filters is carried by a motorized rotating disc to successively bring the filters onto the imager's optical axis. The images of the area observed in the various spectral hands corresponding to the bandwidths of the filters are acquired sequentially and are separated from one another in time by intervals corresponding to the filter changes. These intervals degrade the overall sensitivity of the imager, which is “blind” during filter changes. Moreover, two successive images of the area observed in the same spectral band are separated by a relatively long period of time, corresponding to the time it takes for the filter wheel to complete a full revolution, which affects the temporal coherence of the images processed and poses problems in the event that rapid movements are detected in the observed area.
Furthermore, the means for motorizing the filter wheel are a source of noise and vibration and degrade the imager's overall reliability. In addition, the use of a movable member, namely the wheel, implies that the level of vibrations transmitted to the device must be controlled in order to prevent the vibrations from affecting the operation of the rotating member.
Also known is a filtering device comprising a tube housing a plurality of elongated mirrors along the axis of the tube and arranged side by side about this axis. The luminous flux coming from an observed area of space propagates in the tube and is reflected on the mirrors, which leads to the formation of a plurality of subimages whose number and shape is a function of the number and the arrangement of the mirrors according to the well-known operating principle of a kaleidoscope. An array of photodetectors is arranged at the outlet of the tube in order to collect the various images. It has thus been proposed to arrange a planar filter comprising a plurality of juxtaposed areas intended to each absorb a different frequency band.
While this configuration allows for temporal filtering to be performed in a plurality of different frequency bands, it turns out that the use of a planar filter is difficult since it requires focusing means to be added downstream of the filter to focus the filtered light rays on an array of detectors.
The present invention particularly aims to avoid these disadvantages in a simple, effective and economical manner.